Secondary batteries have been used in various products in recent years. A secondary battery includes a battery element formed by stacking positive electrodes, separators and negative electrodes. In order to form the battery element, the electrodes and the separators are stacked alternately in the order of a positive electrode, a separator, a negative electrode and a separator, for example.
When the separator is stacked on the electrode, the separator is likely to form wrinkles, or to form swells which may develop into wrinkles later. Wrinkles of the separator, if any, make the stacking inhomogeneous, resulting in local application of pressure and variations in the distance between the electrodes. This makes the battery quality worse. For this reason, whether a wrinkle is formed or not is important for evaluating the battery quality. Although the existence of wrinkles can be visually determined, there is still a risk of overlooking wrinkles. In addition, the visual determination is undesirable from a cycle-time viewpoint.
Against this background, a technique has been known in which: a laser beam is emitted onto the surface of the separator; and wrinkles are detected in accordance with the intensity of light reflected off the surface (see Patent Literature 1).